When a battery supplies energy to the drive system in a hybrid automobile or the like, generally a voltage of around several hundred V is required. The battery is often configured by connecting many secondary cells, such as lithium ion batteries, in series. When many secondary cells are connected in series, the voltage varies because the individual cells have different characteristics. Then when charging, for example, the voltage in some cells may be excessive, and may cause problems such as damage or fire. For this reason, circuitry is generally provided to sense the voltage of each of the secondary cells connected in series to prevent an overvoltage.
The voltage sensing device described in FIG. 6 in Japanese Kokai Patent Application No. 2001-264366, for example, is provided with a multiplexer that selects two nodes via the individual connection nodes of the multiple cell modules connected in series, a voltage sensing differential amplifier that senses the voltage at the two nodes selected by the multiplexer, and a module voltage correcting part that corrects the voltage sensed by the voltage sensing differential amplifier based on a prescribed correction formula.
With the voltage sensing device described in Japanese Kokai Patent Application No. 2001-264366, a node of each cell module is first selected by the multiplexer from the multiple cell modules connected in series, and the voltage of the selected cell module is sensed by the voltage sensing differential amplifier. The sensed value is recorded in the module voltage correcting part. Then, when the sensed voltage values for all cell modules have been obtained, the accurate voltage for each cell module is calculated using the prescribed correction formula.
With the voltage sensing device disclosed in Japanese Kokai Patent Application No. 2001-264366, a constant must be obtained before the product is shipped in order to perform correction calculations based on a correction formula in the module voltage correcting part. That is, during preshipping checking, a reference voltage source is connected in place of the cell modules and its voltage sensed; the necessary constant is then calculated with a prescribed algorithm based on the sensed voltage value obtained. Therefore, much time is needed for preshipping checking, and there is also the problem of repeating the same procedure if a shipped device must be later replaced. There is also the problem of increased error due to changes over time if the constant obtained at the time of shipment continues to be used.
In addition, with the voltage sensing device shown in Japanese Kokai Patent Application No. 2001-264366, it is necessary to sense the voltage of each individual cell module. Because processing of a module voltage correcting part is relatively complicated, in Japanese Kokai Patent Application No. 2001-264366, it is realized with a microcomputer. Then, to obtain an accurate voltage value for each cell module, it is necessary to follow these steps: each sensed value of the cell modules undergoes analog-digital conversion, the obtained data are transmitted to the microcomputer, and correction calculations are executed based on the data transmitted to the microcomputer. That is, there is the problem that a large amount of time is required until voltage values are ascertained. Monitoring the cell module voltage to detect abnormalities, such as overvoltages, lead to the possibility that such a time delay may become a problem from the aspect of safety. The use of a microcomputer creates the problem of increased circuit area and power consumption.